The present invention relates to a process for producing flat heat exchange tubes, more particularly to flat heat exchange tubes for condensers, evaporators and like heat exchangers for use in car coolers.
JP-B No. 45300/91 discloses a condenser for use in car coolers which comprises a pair of headers arranged at right and left in parallel and spaced apart from each other, parallel flat heat exchange tubes each joined at its opposite ends to the two headers, corrugated fins arranged in an air flow clearance between adjacent heat exchange tubes and brazed to the adjacent tubes, an inlet pipe connected to the upper end of the left header, an outlet pipe connected to the lower end of the right header, a left partition provided inside the left header and positioned above the midportion thereof, and a right partition provided inside the right header and positioned below the midportion thereof, the number of heat exchange tubes between the inlet pipe and the left partition, the number of heat exchange tubes between the left partition and the right partition and the number of heat exchange tubes between the right partition and the outlet pipe decreasing from above downward. A refrigerant flowing into the inlet pipe in a vapor phase flows zigzag through the condenser before flowing out from the outlet pipe in a liquid phase. Condensers of the construction described are called parallel flow or multiflow condensers, realize high efficiencies, lower pressure losses and supercompactness and are in wide use in recent years in place of conventional serpentine condensers.
It is required that the heat exchange tube for use in the condenser have pressure resistance since the refrigerant is introduced thereinto in the form of a gas of high pressure. To meet this requirement and to achieve a high heat exchange efficiency, the heat exchange tube is made of a hollow aluminum extrudate which comprises flat upper and lower walls, and a reinforcing wall connected between the upper and lower walls and extending longitudinally. To improve the heat exchange efficiency and to compact the condenser, it is desired that the flat heat exchange tube have a small wall thickness and the lowest possible height. In the case of extrudates, however, the extrusion technique imposes limitations on the reduction in the height of the tube and in the wall thickness.
To overcome this problem, a process is known, as disclosed in JP-A No. 281373/97, for producing a flat heat exchange tube having parallel fluid passages in its interior and comprising upper and lower walls, right and left side walls interconnecting the right and left side edges of the upper and lower walls and a plurality of reinforcing walls connected between the upper and lower walls, extending longitudinally of the tube and spaced apart from one another, by brazing a first tube component member of aluminum and a second tube component member of aluminum into an integral assembly, the first component member including a lower wall forming portion and a plurality of reinforcing wall forming portions integral with and extending upward from the lower wall forming portion, the second component member including an upper wall forming portion comprising a brazing sheet having a brazing material layer over the lower surface thereof for interconnecting opposite side wall forming portions of the first component member.
However, it is likely that the upper edge of the reinforcing wall forming portion 51 of the first component member 50 will not be horizontal but will be inclined, for example, from one end toward the other end as shown in FIG. 20 since the first component member is prepared by rolling an aluminum sheet with upper and lower rolling rolls one of which has annular grooves. When the first component member 50 and the second component member 52 are assembled, therefore, the lower surface of the upper wall forming portion 53 of the second component member 52 is in contact with the upper edge of the reinforcing wall forming portion 51 of the first component member 50 only at one end portion of the assembly, with a small clearance 54 created at the other portion. This gives rise to the following problem. When melted for brazing, the brazing material in the form of the brazing material layer 55 over the lower surface of the upper wall forming portion 53 collects to the portion where the lower surface of the upper wall forming portion 53 is in contact with the upper edge of the reinforcing wall forming portion 51 and subsequently flows into the clearance 54 to progressively fill the clearance. Nevertheless, the molten brazing material is not very smoothly flowable due to the influence of an oxide film present over the surface of the brazing material layer 55, consequently failing to completely fill up the clearance 54 and to form a fillet over the entire length of the reinforcing wall forming portion 51.
FIG. 21 shows a case wherein each of reinforcing wall forming portions 51 has a plurality of cutouts 56 formed in its upper edge and arranged at a spacing longitudinally thereof, and the openings of the cutouts 56 are closed with an upper wall forming portion 53 when a first component member 57 and a second component member 52 are brazed to form communication holes for causing parallel refrigerant passages to communicate with one another. Especially in this case, it is likely that the upper edges of the parts 58A, 58B, 58C of the reinforcing wall forming portion 51 of the first component 57 between the adjacent cutouts 56 in the porion 51 will not always be positioned at the same level but will be at different levels. It therefore follows that the upper edge of only one part 58A is in contact with the upper wall forming portion 53 of the second component member 52, with a small clearance 59 formed between the upper wall forming portion 53 and the upper edges of the other parts 58B, 58C. When the two component members are brazed, the upper edges of the reinforcing wall forming portions 51 of the first component member 57 remain unbrazed to the upper wall forming portion 53 of the second component member 52 at the locations where the clearance 59 exits. The brazing operation consequently fails to give sufficient strength to the joints between the upper wall forming portion 53 of the second component member 52 and the reinforcing wall forming portions 51 of the first component member 57, and the flat heat exchange tube produced is unable to fulfill the pressure resistance requirement.
It appears useful to give an increased thickness to the brazing material layer over the lower surface of the second component member in order to solve the problem, but the brazing material then drips during brazing to reduce the cross sectional area of the fluid passages to result in increased resistance to the flow of the fluid, possibly clogging up the fluid passage. In the case where the reinforcing wall forming portions have cutouts, there is the likelihood of the brazing material closing communication holes.
An object of the present invention is to provide a process for producing flat heat exchange tubes free of the foregoing problems relating to brazing and having satisfactory pressure resistance.